1. 重复键的去除。

Map<Long, Long> restaurantMap = restaurants.stream().collect(Collectors.toMap(Restaurant::getElemeRestaurantId, Restaurant::getStoreId, (key1, key2) -> key2));

2. List 以ID分组 Map<Integer,List<Apple>>

Map<Integer, List<Apple>> groupBy = appleList.stream().collect(Collectors.groupingBy(Apple::getId)); 

3. List以ID分组后聚合对象的一个属性

        Map<Long, List<Long>> restaurantMap = restaurants.stream().collect(Collectors.toMap(
                Restaurant::getStoreId,
                r -> Collections.singletonList(r.getElemeRestaurantId()),
                (x, y) -> {
                    List<Long> l = new ArrayList<>();
                    l.addAll(y);
                    return l;
                }
        ));